Seismo‐Mechanical Response of Anisotropic Rocks Under Hydraulic Fracture Conditions: New Experimental Insights

Abstract

Unconventional hydrocarbon resources found across the world are driving a renewed interest in mudrock hydraulic fracturing methods. However, given the difficulty in safely measuring the various controlling factors in a natural environment, considerable challenges remain in understanding the fracture process. To investigate, we report a new laboratory study that simulates hydraulic fracturing using a conventional triaxial apparatus. We show that fracture orientation is primarily controlled by external stress conditions and the inherent rock anisotropy and fabric are critical in governing fracture initiation, propagation, and geometry. We use anisotropic Nash Point Shale (NPS) from the early Jurassic with high elastic P wave anisotropy (56%) and mechanical tensile anisotropy (60%), and highly anisotropic (cemented) Crab Orchard Sandstone with P wave/tensile anisotropies of 12% and 14%, respectively. Initiation of tensile fracture requires 36 MPa for NPS at 1‐km simulated depth and 32 MPa for Crab Orchard Sandstone, in both cases with cross‐bedding favorable orientated. When unfavorably orientated, this increases to 58 MPa for NPS at 800‐m simulated depth, far higher as fractures must now traverse cross‐bedding. We record a swarm of acoustic emission activity, which exhibits spectral power peaks at 600 and 100 kHz suggesting primary fracture and fluid‐rock resonance, respectively. The onset of the acoustic emission data precedes the dynamic instability of the fracture by 0.02 s, which scales to ~20 s for ~100‐m size fractures. We conclude that a monitoring system could become not only a forecasting tool but also a means to control the fracking process to prevent avoidable seismic events.

Fluid‐Driven Tensile Fracture and Fracture Toughness in Nash Point Shale at Elevated Pressure

“…The axial principal stress, σ 1 , is controlled via electronic servo‐control to ensure positive sealing throughout the experiment. After Gehne et al ().…”

“…Our test material was Nash Point shale. This material is known to be highly anisotropic and is considered to be representative of the type of mudrocks being targeted as unconventional hydrocarbon reservoir materials (Forbes Inskip et al, ; Gehne et al, ). Nash Point shale is the shaly member of the Porthkerry formation, which is Hettangian‐Sinemurian in age and outcrops at Nash Point, South Wales, UK.…”

“…A detailed description of the material is given in Forbes Inskip et al (). The mechanical and P ‐wave anisotropy is 60% and 56%, respectively, as measured from indirect tensile strength tests and radial P ‐wave velocities (Gehne et al, ). It has a porosity of approximately ~6.5%, measured by helium pycnometery, and an intact permeability of approximately 10 −18 m 2 parallel to bedding and 10 −20 m 2 normal to bedding (Gehne & Benson, ), also measured using helium gas.…”

“…To account for material anisotropy, samples were prepared with axes either parallel or normal to the bedding planes. The sample assembly is separated from the confining pressure medium using an engineered nitrile jacket fitted with ports for up to 16 measurement sensors (11 acoustic emission [AE] sensors, one borehole pressure sensor, and four radial deformation sensors) (Gehne et al, ). The central borehole was fitted with two internal steel guides, sealed with respect to the borehole wall using a number of O‐rings (Figure ).…”

“…Driven by this lack of consistency, and the inherent problems associated with sealing notched fracture mechanics samples, we present new data from a novel technique used to calculate K Ic from tests using internally pressurized thick‐walled cylinder samples (Abou‐Sayed, ; Gehne et al, ).…”